We propose the creation of a wholly novel technology protein painting, for the rapid, direct isolation and sequencing of hidden native protein-protein interaction domains. Protein-protein interactions are the basis of virtually all functional moleculr events driving cancer cell signaling and gene regulation. Currently no technology exists to directly isolate and sequence unknown interaction domains, or to directly detect whether known protein-protein binding domains are in contact in a cellular or tissue sample of native proteins. Tracking the interface domains between interacting proteins, or within misfolded proteins, is the basis for the next generation of therapies that block the molecular interactions driving cancer. We will create protein paint chemistries, a novel panel of synthetic organic small molecules that bind to protein molecules with high affinity and mask all the protease cleavage sites of the protein. We will use our new protein painting chemistry to isolate and sequence protein-protein interaction domains of cancer related proteins. Interacting native proteins in solution are painted with a palette of paint molecules that coat the exposed surfaces of the proteins but do not have access to the internal protein-protein contact domains. Thus if two native proteins are bound together, the interface domains will remain non-painted. The protein paints coat with a high resolution (<3 amino acids). Following painting, the interacting proteins are dissociated, to reveal and expose the non-painted interaction domains that were inaccessible to the paint molecules when the proteins were interacting in their native state. Painted regions are masked from proteinase cleavage or antibody recognition, even after dissociation. The dissociated painted proteins can then be subjected to proteinase cleavage (e.g. trypsin) and ms sequencing, or antibody probing. Since the paint blocks the proteinase cleavage sites that are not in contact, proteinase fragments for ms sequencing will only be generated from the non-painted areas exclusively comprising the interaction domains. The Aims follow our discovery of the masking of trypsin cleavage sites (carboxyl side of arginine & lysine) by sulfonated anthracene organic dye molecule paints such as disodium 1-amino-9,10- dioxo-4- [3- (2- sulfonatooxyethylsulfonyl) anilino] anthracene -2 -sulfonate, which achieve a high degree of trypsin cleavage site coverage. We verified protein painting capabilities by direct ms sequencing of the hidden interaction domain of interleukin-1 bound to its receptor. In the past this domain could only be predicted by X- Ray crystallography. Our transformative technology addresses a broad and critical unmet need in cancer biology and cancer therapeutics. We envision that the technology can be used in the future to decipher interaction domains from tumor biopsy samples or cell cultures treated with a ligand or a therapy. Antibody binding assays can be used to quantify the number/type of occupied binding sites in a cell lysate. We will create a panel of 12 protein paints, and apply the paints to 3 model cancer related protein-protein interactions:1) receptor-ligand, 2) phosphorylation mediated association, and 3) transcription factor complexes.